[Winga]

How many stereogenic centers are there in xylitol?

[movies]

You tell me.  

[Winga]

I know the answer, but I want someone to answer first!

[Mitch]

I know the answer, but I want someone to answer first!

Not the way it works.

[Winga]

3, and it is achiral.

[movies]

Achiral compounds can still have stereocenters!

[Winga]

There is a pair of enantiomeric substituents (R & S) attached to the central carbon.
If we now change thenm to either 2S or 2R, the molecule will become chiral but the central carbon is not a stereocenter (although 2 identical substituents attach to it).

[movies]

Be careful with your terminology.  The central carbon (C3) is indeed not a stereocenter in the natural form, it would become a stereocenter if you were to change the configuration at either C2 or C4.  I think that this is what you were suggesting, Winga.

The tricky part of it is that if you change the configuration at C3, you still have an achiral compound, but it is a different diastereomer.  I believe the correct term for a carbon center which is not a stereocenter but leads to a different diastereomer when inverted is achirotopic.  It's a consequence of the plane of symmetry that intersects the C3 atom.

If you want to learn more, I recommend this book:

http://www.amazon.com/exec/obidos/tg/detail/-/0471016705/

[Winga]

I don't agree with you, movies.

The natural of xylitol, the carbon 3 is attached by two stereogenic groups (also H & OH), one is (R)configuration while another one is (S) form, you can figure them out.

(R) and (S) are assigned to two different groups, so, the carbon 3 is attached to 4 different groups and it is a stereocenter. However, it is achiral due to the plane of symmetry.

By changing one of the stereogenic groups, e.g. (S) -> (R), now, the carbon 3 is attached by 2 identical groups, 2 (R) groups. Therefore, it is not a stereocenter anymore, but it is chiral, no plane of symmetry.

[Mitch]

By changing one of the stereogenic groups, e.g. (S) -> (R), now, the carbon 3 is attached by 2 identical groups, 2 (R) groups. Therefore, it is not a stereocenter anymore, but it is chiral, no plane of symmetry.

Not true, movies is right. Never use the absence or presence of a plane of symmetry to determine if chirality exists. Only seeing if the mirror image is superimposable will determine chirality with 100% accuracy.

[Winga]

Not true, movies is right. Never use the absence or presence of a plane of symmetry to determine if chirality exists. Only seeing if the mirror image is superimposable will determine chirality with 100% accuracy.

The mirror image of the (2R,4R)-molecule is (2S,4S). They are not superimposable. Both of them are chiral.

[movies]

The mirror image of the (2R,4R)-molecule is (2S,4S). They are not superimposable. Both of them are chiral.

That's true, but in xylitol it is (2R,4S), which is achiral.

[Garneck]

And that's why I don't like organic chemistry